This one envisages Bran in a future where Westeros has been entirely destroyed by White Walkers. However, through his connection to the weirwood tree, he's able to see an alternate timeline, where things work out better (as 'better' as things get in Westeros). Thus he goes back in time, wargs into Jaime, and has him push young Bran out the window, thus starting the Game of Thrones but saving the Seven Kingdoms.
5. He Is The Three-Eyed Raven
This theory posits that Bran warged into Brynden Rivers of the past (the Three-Eyed Crow from the books, though the show is less explicit in revealing his true identity), and then controlling him makes him go to the weirwood tree, become the Three-Eyed Raven, and then start communicating with Bran. This leads Bran on a path to the tree, where he learns from the Raven, and then leaves the tree as we've seen in the show.